Actually, Allen's had it for a few months by now, considering how long Lenalee's hair has grown. Yes, he only used it against two Noah, and there may have been other reasons behind its failure to injure Road, but that's still a fairly unimpressive record. After all, there are only four (five?) living Noah that we've seen right now. Anyway, it *is* pretty epic as a weapon, and he does use it to kick ass - it's not worthless by any means. It just might not do the job it should have done.
(Nonono, it's fine, I READ the spoiler so I meant that I had an idea of what was going on.)